In downhole drilling operations, downhole measuring tools are used to gather information about geological formations, status of downhole tools, and other downhole conditions. Such data is useful to drilling operators, geologists, engineers, and other personnel located at the surface. This data may be used to adjust drilling parameters, such as drilling direction, penetration speed, and the like, to effectively tap into an oil or gas bearing reservoir. Data may be gathered at various points along the drill string, such as from a bottom-hole assembly or from sensors distributed along the drill string. Once gathered, apparatus and methods are needed to rapidly and reliably transmit the data to the surface. Traditionally, mud pulse telemetry has been used to transmit data to the surface. However, mud pulse telemetry is characterized by a very slow data transmission rate (typically in a range of 1-6 bits/second) and is therefore inadequate for transmitting large quantities of data in real time. Other telemetry systems, such as wired pipe telemetry system and wireless telemetry system, have been or are being developed to achieve a much higher transmission rate than possible with the mud pulse telemetry system.
Wired pipe telemetry systems using a combination of electrical and magnetic principles to transmit data between a downhole location and the surface are described in, for example, U.S. Pat. Nos. 6,670,880, 6,992,554, and 6929493. U.S. Pat. No. 6,670,880, for example, discloses that such a system will transmit data at a rate of least 100 bits/second and conceivably at a rate as high as 1,000,000 bits/second. In these systems, inductive transducers are provided at the ends of wired pipes. The inductive transducers at the opposing ends of each wired pipe are electrical connected by an electrical conductor running along the length of the wired pipe. Data transmission involves transmitting an electrical signal through an electrical conductor in a first wired pipe, converting the electrical signal to a magnetic field upon leaving the first wired pipe using an inductive transducer at an end of the first wired pipe, and converting the magnetic field back into an electrical signal using an inductive transducer at an end of the second wired pipe. Several wired pipes are typically needed for data transmission between the downhole location and the surface.
For uninterrupted data transmission from the downhole location to the surface, the transducer devices used in the wired pipe telemetry system must be electrically and structurally reliable. Several measures have been taken to ensure electrical reliability of inductive transducers. U.S. Pat. No. 6,992,554, for example, describes a robust data transmission element (i.e., inductive transducer) for transmitting information between downhole components. In this patent, the data transmission element includes a U-shaped annular housing. A U-shaped magnetically conducting, electrically insulating (MCEI) element is arranged in the U-shaped annular housing. An insulated conductor is located within the U-shaped MCEI element. As current flows through the insulated conductor, a magnetic flux or field is created around the insulated conductor. The MCEI element contains the magnetic flux created by the insulated conductor and prevents energy leakage into surrounding materials. The annular housing is made of a hard material that is electrically conductive, typically a metal. Although not specifically discussed in this patent, there is a through-hole in the annular housing as well as the MCEI element to allow for insertion of an input lead to the insulated conductor. Thus, a weak spot is inherently designed into the annular housing.
U.S. Pat. No. 6,992,554 discloses that the annular housing stretches as it is forced into the recess within the mating surface of a downhole component. This stretching action provides a rebound force to return the annular housing to its original position when the force is removed. When the annular housing stretches, the area surrounding the through-hole created in the annular housing for the input lead would absorb more of the stretch than the rest of the annular housing. As a result, strain induced in the annular housing as a result of the stretching would concentrate around the through-hole for the input lead. The material in this highly strained region may exceed its elastic limit sooner than the material in the remainder of the annular housing, causing the annular housing and inductive transducer to fail structurally prematurely. This disclosure discloses how to prevent or curb this premature structural failure.